Conventional blind fasteners generally do not have the tension or shear capabilities achievable with conventional solid bolts because the blind fasteners include an internal screw, pull stem, or other detail in the shear plane to disturb the principal load carrying paths in shear or tension. Conventional solutions to achieve comparable shear and tension performance usually involve substituting exotic high strength materials, such as Inconel, for the titanium or steel alloys commonly used in fasteners for primary aerospace structure. These exotic materials introduce cost and weight penalties and may pose corrosion problems, and still have inferior fatigue endurance because of the internal threads or other features. The key factors in fatigue endurance are pre-load (clamp-up) and absence of fatiguable features in the shear plane. The clamp up force generates friction between the elements so that they resist movement. Some blind fasteners have removed details, like threads, from the shear plane, but, in doing so, have sacrificed pre-load capacity because they have removed any threaded portions from the fastener. Therefore, such blind fasteners also have inferior fatigue endurance.
The blind fastener industry today has two basic fastener types. With a screw type blind fastener, you achieve excellent pre-load using a buckling upset sleeve approach by turning a threaded core bolt in a drive nut. Fasteners of this screw type are illustrated in U.S. Pat. Nos. 4,858,479; 4,457,652; 4,579,491; 4,659,271; 4,772,167; 4,900,205; 4,919,577; 4,950,115; 4,967,463; 4,968,198; 5,066,169; and 5,238,342 which we incorporate by reference. In the screw type blind fastener, threads extend across the shear plane.
Pull type blind fasteners use a solid shank pin inside a hollow shank bolt. The shear plane may lack any fatigue critical details, like threads, but the conventional designs that rely on pulling force for pre-load provide inferior pre-load than achievable with screws or bolts. Pull type blind fasteners are described in U.S. Pat. Nos. 4,089,247; 4,142,489; and 4,865,499 which we incorporate by reference. The absence of adequate pre-load means that the joint can move too easily along the axis of the core bolt and such a joint remains prone to fatigue.
Blind fasteners have found almost no acceptance in aerospace primary structural assemblies because of their inferior fatigue endurance. In previous attempts to reduce localized stress at the thread root, fastener manufacturers have tried to improve the thread detail in the bolt body to a sinusoidal profile or to increase the wall thickness of the outer bolt to reduce field stress at the thread root. These attempts failed to increase the fatigue endurance adequately. Therefore, there remains a need for a blind structural fastener that will provide fatigue endurance comparable to a conventional bolt. The need is magnified by automated assembly processes that would greatly benefit from the ability to set fasteners with access to only one side. The present invention describes such a structural blind fastener.